Oils for lubricating engines, such as internal combustion engines, are formulated with a combination of additives for improving and prolonging oil performance. The additives counter oil degradation that occurs during use. For example, thermoplastic polymers with antioxidant additives have been incorporated into engine oil to extend the useful life of the oil, as described in U.S. Pat. Nos. 4,066,559 and 4,144,166, the disclosures of which are each incorporated herein by reference. Over time, however, the additives are depleted during use of the oil. Oil degradation and sludge formation occurs due to the oxidative deterioration of engine oil at high temperatures and reaction between the engine oil and fuel, water, blow-by gas (constituents comprising O2, N2, NOx, SOx), or by other means. The trend in recent years has been toward the higher output of a gasoline engine and smaller capacity of an oil pan for the engine oil to save energy, reducing the amount of oil and, at the same time, the amount of additives in the oil.
One proposed solution to depletion of oil additives has been to provide a source for replenishing the additives. U.S. Pat. Nos. 5,591,330 (Lefebvre), 5,552,040 (Baehler et al.), 4,075,097 (Paul), and 4,075,098 (Paul et al.) describe oil filters modified to release additives into the engine oil over time. U.S. Pat. No. 5,718,258 to Lefebvre et al. describes a separate canister for releasing oil additives into oil that is mounted between the oil filter and the engine block. The methods described in these patents, however, can provide only limited amounts of oil additives. Further, the methods do not contemplate non-linear rates of additive release. Non-linear rates of additive release would be desirable, for example, to adjust additive release to engine performance or to a desired replenishment plan for a particular vehicle. For these reasons, there remains a need for efficient and effective replenishment of oil additives into engine oil.